1. Field of the Invention
The invention relates to scanning antennas in general and particularly to space-fed phased arrays.
2. Description of the Prior Art
Since the original development of techniques for inertialess scanning, there has been steady development in antenna systems providing the rapid scanning and random beam pointing basically made possible thereby. The various types of so-called phased-arrays usually contemplate a two-dimensional planar or curved surface arrangement of antenna elements (radiators) arranged to be fed and phased-controlled individually or in groups to form a beam in space and control its pointing angle on an instantaneous basis.
Chapter 11, entitled "Array Antennas" in the textbook entitled, "Radar Handbook" by Merrill I. Skolnik (McGraw-Hill 1970) is a useful and relatively current reference for obtaining an understanding of the state of the prior art in phased-array systems.
Of particular interest as prior art for the present invention, is Section 11.7 of that reference, which discusses optical (space) feed systems.
The various arrangements for feeding the elements of the phased-array have advantages and disadvantages, an the selection of a type of feed to be used in a system designed consideration. The so-called space or optical feed, phased-array system is the one to which the present invention applies. That type of feed employs a primary source of feed which may be in the form of a horn, or the like, arranged to illuminate the "back" of a panel or surface of the array which may be referred to as the rear aperture thereof. There is an intervening space between the primary feed and the array in such configurations, hence, the term space feed.
In such arrangements, the array itself may be thought of as a lens. Each of the radiators or antenna elements is essentially a feed through device which intercepts a portion of the primary feed illumination, subjects it to a controlled amount of phase delay and re-radiates it through the front aperture surface of the array. Programming the individually controllable phase shifters within the plural antenna elements thereby provides the desired beam pointing.
One particular advantage assignable to space-fed arrays is the relatively simple nature of the feed. The horn or other primary feed device may be readily designed to distribute the radiatable power uniformly over the rear array aperture or to provide whatever aperture tapering is desired, for sidelobe control or other purposes. To accomplish the same distributed feed by means of corporate feed techniques requires substantial additional hardware, a fact recognized by those skilled in this art.
The aforementioned "Radar Handbook" reference teaches that the space-fed phased array may be constructed to function in the refractive (transmission) lens mode or alternatively it may be constructed so that the individual antenna elements operate in a reflective mode so that the array is essentially a reflecting surface capable of individually controlling phases over its surface. In either case, controllable phase shifters are capable of providing the beam pointing function. The individual antenna elements may be thought of as refractive cells, and as such each is a 2-port device having a rear radiator, a phase shifter and a front radiator.
In the known prior art reflective mode, the antenna elements are individual refractive 1-port devices including a radiator and a phase shifter followed by a short or open circuit arrangement (depending upon the specific microwave parameters) to provide a reflecting point such that energy intercepted by the radiator passes through the phase shifter on the way to the reflective point and also on the way back. Such a reflective mode arrangement obviously requires the use of a reciprocal type phase shifter. In a transmission-type phased-array antenna of the type above mentioned it is often highly desirable to be able to provide some rearward detection capability. Some prior art approaches for dealing with that problem, involve such expedients as replacing some of the phased shifter elements with passive reflecting shorts. Such a scheme has severe limitations in that the rearward beam cannot be steered, its shape is not subject to programming, the gain in radiated power of the rearward beam are necessarily minimal. Still further, the ratio of forward-to-rearward radiated power cannot be electronically controlled.
Quite obviously, a fully operative and maximally flexible system can be provided by employing either two lens type (forward transmission) arrays with space feeds essentially back-to-back or with two reflective type arrays similarly back to back. In that way, fully programmed beam pointing can be afforded over substantial forward and rearward solid angles. However, the diseconomy of such an approach is obvious.
The matter in which the present invention deals with the prior art limitations aforementioned, by means of a novel combination will be understood as this description proceeds.